1. Technical Field
The present invention relates to wire used in biomedical applications and, in particular, relates to a biodegradable composite wire for use in medical devices such as stents.
2. Description of the Related Art
Stents are artificial tube-like structures that are deployed within a conduit or passage in the body to alleviate a flow restriction or constriction. Stents are commonly used in coronary arteries to alleviate blood flow restrictions resulting, e.g., from cardiovascular disease. However, stents may also be used in non-coronary vessels, the urinary tract and other areas of the body. Non-coronary applications range broadly from compliant pulmonary vessels of children with congenital heart disease (CHD), to atherosclerotic popliteal arteries of older patients with critical limb ischemia (CLI). Stented lesions may be long and tortuous as in the case of severe infrainguinal lesions, or short and relatively uniform as in mild pulmonary artery stenoses.
Examples of non-coronary stent applications include arteriovenous fistulas (AVFs) or false aneurysms, which may occur as a result of trauma due to gunshot wounds, falling accidents, or other blunt force incident. Such phenomena often occur in the upper limbs of the body where lack of perfusion can manifest as gangrene, severe pain, or local cyanosis. Critical limb ischemia associated with atherosclerosis can also result in the need for radial or axillary artery stenting, for example, to avoid amputation or other more serious morbidities. In contrast to most thoracoabdominal implantation sites (such as in coronary arteries), upper and lower limb anatomy is typically subjected to greater range of motion, thereby potentially increasing mechanical fatigue.
Typically, stents are made of either biocompatible metal wire(s) or polymeric fiber(s) which are formed into a generally cylindrical, woven or braided structure of the type shown in FIGS. 1A and 1B. These types of stents are typically designed to be either “self-expanding”, in which the stent may be made of a shape memory material, for example, and deploys automatically by expanding upon removal of a constricting force when released from a containment device, or “balloon-expanding”, in which the stent is forcibly expanded from within by an inflatable balloon.
When a stent is implanted, it applies a radial force against the wall of the vessel in which it is implanted, which improves vessel patency and reduces acute closure or increases vessel diameter. In either case, the vessel usually achieves a new equilibrium by biological remodeling of the vessel wall over a period of weeks or months. After such remodeling is complete, the stent may no longer be needed for mechanical support and could potentially inhibit further natural positive remodeling of the vessel or limit re-intervention, for example. However, removal of the stent may be difficult.
Many known stents are formed of corrosion-resistant and substantially non-biodegradable or non-bioresorbable metal materials which maintain their integrity in the body for many years after implantation. Design efforts for creating bioabsorbable stents have focused primarily on balloon-expandable technology for coronary pathologies, and may include polymer biodegradable stents using poly-L lactic acid (PLLA) and poly-L glycolic acid (PLGA), nutrient metals of magnesium (Mg), including alloys or powder metallurgy forms of magnesium, and iron (Fe), and iron-manganese alloys. Some research methods have also focused on hybrids including layered biodegradable polymers and bioabsorbable polymer coated nutrient metals. While such materials are resorbable, they may have low mechanical strength and resilience, and/or may confer inadequate control over the rate of bioabsorpotion (i.e., by biodegrading too slowly or quickly in vivo).
What is needed is a biodegradable metallic wire with sufficient mechanical properties and appropriate degradation rates for use in biomedical applications, which represents an improvement over the foregoing.